A device for detecting when a limit load is bearing on a crane arm

Abstract: The invention relates to a device for detecting when a limit load weight bearing on a crane arm has been reached. The device comprises a cursor (6), which is slidingly mobile within a seating (8) and which is provided on opposite sides with a first end (6a) and a second end (6b) on which fluids of two pilot conduits (4 and 5) act reciprocally contrastingly. Each of the pilot conduits (4 and 5) communicates with one of the two opposite chambers of a double-acting cylinder which activates a crane arm. The invention is of particular application in blocking manoeuvring of a crane when such manoeuvring would cause an increase in a load bearing on the crane arm that would reach a load limit.

Description: The invention relates specifically but not exclusively to a device for blocking manoeuvring of a crane when a load on said arm reaches a load limit level. The invention especially relates to a device comprising: a) a balance valve, arranged on a hydraulic transmission line of a double-acting cylinder provided with a first chamber on the cylinder base side associated to the raising of a crane arm and a second chamber, on the stem side of the cylinder, associated to the lowering of the arm; b) a first pilot conduit, communicating with the first chamber of the cylinder; c) a second pilot conduit, communicating with the second chamber of the cylinder; d) a cursor, slidingly mobile within a seating and provided on opposite sides with a first end and a second end on which the fluids of the first and second pilot conduits act reciprocally contrastingly, the cursor being pushed into a block position whereat it activates a cylinder block actuator by effect of the action of the fluid of the first pilot conduit; e) a spring, acting on the cursor contrastingly to the action of the fluid in the first pilot conduit. The cursor moves into the block position when the force exerted by the pressurized fluid on the first end overcomes the spring force added to the force exerted by the fluid on the second end. In a correct functioning of the device the difference between the force exerted by the pressurized fluid on the first end and the force exerted by said fluid on the second end must be proportional to the load bearing on the arm. If this is true, it is sufficient to calibrate the spring to a value which is proportional to the limit load on the arm, and when such load limit is reached the cursor automatically moves into a block position. In such devices the above-mentioned proportional rapport is conserved also during the arm lowering phase; that is, when, due to the necessity of piloting the balance valve, the pressure inside the second chamber of the cylinder increases and, consequently, so does the pressure in the first chamber correspondingly. The increase in the apparent pressure (that is, pressure increase not due to the load on the crane) in the first chamber is compensated by causing not only the pressure signal coming from the first chamber to act on the cursor, but also the counterpressure signal coming from the second chamber through the second pilot conduit to follow suit. Without this consideration, during a lowering of the arm the crane commands would block, and not due to the load limit having been reached. Various limiters of this type have been disclosed, in which however the relation among the various areas of the cylinder piston surface area on which the fluid acts on the stem side and the base side must be exactly equal to the relation among the surface areas of the two ends of the cursor, on which the fluid acts. If this is not so there cannot be a perfectly proportional relation between, on the one hand, the difference between the two forces exerted by the pressurized fluid on each end of the cursor and, on the other hand, the effective load bearing on the arm. The device is thus constructionally complicated and expensive, as both the cursor and the seat must be made to extreme levels of precision and further are only suitable for a cylinder having a predetermined stem-piston size rapport. One solution to this problem is described in patent DE-A-1926236, in which the cursor is in fact the piston of a second, small cylinder, and in which a pressure regulator is provided on the second pilot conduit and has the function of limiting the fluid pressure downstream of the regulator to a maximum predetermined value, or of reducing the pressure downstream by a constant quantity with respect to the upstream pressure, independently of the changes in the latter. This pressure regulator serves to correct the pressure in the second pilot conduit, as the stem/piston size rapport is greater in the case of the activating cylinder of the arm than in that of the small activating cylinder for the crane arm block. This solution does not however completely solve the problem, since the correction effected by the pressure regulator, due exactly to the way in which it acts, is effective only within a restricted pressure range in the second chamber. As it gradually leaves that range, the difference in the forces due to the pressurized fluid and acting on the two opposite sides of the small piston is ever less proportional to the load bearing on the arm. The main aim of the present invention is to offer a solution to the above-described problem by providing a device which is constructionally simple and effective, in which the above-mentioned force difference is always proportional to the load bearing on the arm in all possible load conditions, and for whatever value of the second chamber pressure; and which is easily adaptable for cylinders having any stem-piston size relation. One advantage of the invention is that it can be practically applied without any special modifications to cranes at present in use. These aims and advantages and others besides are all achieved by the device of the invention, as it is characterized in the accompanying claims, which comprises a drainage conduit, communicating with a discharge and with the second end of the cursor, and wherein a first and a second orifice, each of which exhibit a predetermined diameter section, are provided on the second pilot conduit and on the drainage conduit. Further characteristics and advantages of the present invention will better emerge from the detailed description that follows, of an embodiment of the invention, illustrated in the form of a non-limiting example in the accompanying drawing, in which figure 1 schematically shows the device applied to a crane. With reference to the figure, the crane 1 is provided with an arm 2. A double-acting cylinder 3 is provided with a first chamber 3a on the cylinder base side, which first chamber 3a is associated to the raising of the arm 2 and is further provided with a second chamber 3b on the stem side, which second chamber 3b is associated to the lowering of the arm 2. A balance valve 14 is arranged on a hydraulic drive line 15 of the double-acting cylinder 3, which hydraulic drive line 15 comprises a pump 16 and a distributor 17. The device comprises a first pilot conduit 4 communicating with the first chamber 3a and a second pilot conduit 5 communicating with the second chamber 3b. The device further comprises a cursor 6 which is slidingly mobile in a constant-section seating 8 and which is provided with a first end 6a and a second end 6b, on which the fluids of respectively the first and the second pilot conduits 4 and 5 act in reciprocal contrast. The surface areas of the ends 6a and 6b which the fluids act upon are identical. The cursor 6 is forced by effect of the action of the fluid from the first pilot conduit 4 towards a block position whereat it activates a block actuator 7. In the illustrated embodiment the cursor 6 regulates the distribution of the flow among three pipes emerging from the seating 8, wherein a first pipe T is connected to a discharge 13, a second pipe S is connected to the hydraulic block actuator 7 and a third pipe P is connected to the first chamber 3a of the cylinder. In the cursor 6 blocked position, the second pipe S is placed in communication with the third pipe P, allowing for the activation of the block actuator 7. A spring 9 acts on the cursor 6 in contrast to the action of the fluid of the first pilot conduit 4. The device, according to the invention, comprises a drainage conduit 10, which communicates with a discharge 13 and with the second end 6b of the cursor 6. A first and a second orifice 11 and 12 are provided, each having a predetermined diameter section, which are arranged respectively on the second pilot conduit 5 and on the drainage conduit 10. Thanks to the two orifices 11 and 12 the fluid pressure acting on the second end 6b is comprised between the external pressure of the discharge 13 and the pressure in the second chamber 3b. The value of this intermediate presure depends on the orifice sections. The orifices give rise to bleeding, by means of which a pressure acting on the second end 6b is always proportional to the pressure in the second chamber 3b. According to the invention the passage sections of the orifices 11 and 12 are such that to an increase in pressure in the first chamber 3a of the cylinder, caused by a proportional increase in the second chamber 3b due to the need to pilot the balance valve 14 during an arm lowering operation, corresponds an equal increase in the pressure of the fluid acting on the second end 6b. In other words, the orifices 11 and 12 are conformed such that the pressure acting on the second end 6b is porportional to the pressure in the second chamber 3b in accordance with the relation between the area of the cylinder piston on the stem side and the area of the same piston on the base side. With the invention it is easy to ensure that the difference in the opposed forces on the cursor 6 due to the fluids acting on the ends 6a and 6b is constantly proportional to the load bearing on the arm in all possible load conditions, and for whatever value of the pressure in the second chamber. On reaching a limit load on the arm 2, the cursor 6 automatically moves into a block position. An advantage of the invention is that it uses a simple concept, that of a cursor and a seating in which said cursor runs. A further advantage of the invention is that the cursor runs in a constant-section seating with almost negligible friction. The device can be advantageously applied on existing-type cranes, whatever the cylinder stem/piston size relation might be. When the relation changes it is sufficient to modify the relation between the passage sections of the orifices 11 and 12, leaving the cursor 6 unvaried.

Claims: A device for detecting when a limit load is bearing on a crane arm, comprising: a balance valve (14), arranged on a hydraulic drive line (15) of a double-acting cylinder (3) provided with a first chamber (3a) on a base side of the double-acting cylinder (3) for raising a crane arm and a second chamber (3b), on a stem side of the double-acting cylinder (3), for lowering the crane arm; a first pilot conduit (4), communicating with the first chamber (3a) of the double-acting cylinder (3); a second pilot conduit (5), communicating with the second chamber (3b) of the double-acting cylinder (3); a cursor (6), which is slidingly mobile in a seating (8) and which is provided oppositely with a first end (6a) and a second end (6b) on which first end (6a) and second end (6b) fluids of the first pilot conduit (4) and the second pilot conduit (5) act reciprocally contrastingly; the cursor (6) being pushed into a blocked position wherein it activates a cylinder block actuator (7) by effect of an action of the fluid of the first pilot conduit (4); a spring (9), acting on the cursor (6) contrastingly to the action of the fluid in the first pilot conduit (4); characterised in that: it further comprises a drainage conduit (10) in communication with a discharge (13) and with the second end (6b) of the cursor (6); a first orifice (11) and a second orifice (12) are afforded respectively on the second pilot conduit (5) and the drainage conduit (10), each of which first orifice (11) and second orifice (12) has a predetermined diameter section. A device as in claim 1, characterised in that: the first end (6a) and the second end (6b) on which the fluids act have identical surface areas; the first orifice (11) and the second orifice (12) have passage sections made in such a way that to an increase in pressure in the first chamber (3a) of the cylinder, caused by a proportional increase in the second chamber (3b) for piloting the balance valve (14) during a lowering phase of the crane arm (2), corresponds an equal increase in pressure of the fluid acting on the second end (6b). A device as in claim 1 or 2, characterised in that: the cursor (6) regulates a distribution of a fluid flow rate among three pipes emerging from the seating (8), wherein a first pipe (T) of the three pipes is connected to the discharge (13), a second pipe (S) of the three pipes is connected to the hydraulic block actuator (7) and a third pipe (P) of the three pipes is connected to the first chamber (3a) of the cylinder; when the cursor (6) is in the blocked position, said second pipe (3) is set in communication with said third pipe (P).

Inventors: STORCI ANDREA, STORCI, ANDREA
Applicants: OIL CONTROL SPA, OIL CONTROL S.P.A.
Application year: 1994
Main classification: B66C23
IPC codes: B66C23